1. Field of the Invention
The present invention relates to a dry developer for developing an electrostatic latent image formed by electrophotography, electrostatic recording, electrostatic printing or the like.
2. Discussion of the Background
Two-component dry developers that include a mixture of a toner and a carrier are well known. In a two-component dry developer, toner particles having a relatively small particle size are retained on the surface of carrier particles having a relatively large particle size by an electric force caused by friction between the toner particles and the carrier particles. When the dry developer approaches an electrostatic latent image, and the force which is caused by an electric field formed by the latent image becomes greater than the binding force between the toner particles and the carrier particles, the toner particles are attracted to the latent image, resulting in visualization of the latent image. The developer may therefore last a long time, requiring only the addition of toner commensurate with the amount of the toner spent for developing latent images.
In a two-component dry developer, metal oxides such as magnetite, ferrite and the like are typically employed as a carrier material. These metal oxides have a relatively small bulk density compared to an iron powder. The metal oxide carriers have the following advantages:
(1) a lightweight dry developer can be obtained by using one of the metal oxide carriers; PA1 (2) a dry developer including one of the metal oxide carriers can be easily agitated in a developing unit because the developer is lightweight; PA1 (3) the metal oxide carriers can maintain good magnetic properties such as low remanent magnetic flux density, low coercive force and a small area hysteresis loop regardless of its magnetization history; and PA1 (4) the metal oxide carriers are chemically stable and are relatively unaffected by chemicals such as ozone, nitrogen oxides (NO.sub.x) and the like, which tend to be formed in copying machines. PA1 a toner and a resin-coated carrier, wherein PA1 mixing a toner and a resin-coated carrier, wherein PA1 a charge controlling agent that includes an aromatic hydroxycarboxylic acid metal salt having the following formula (1): ##STR5## wherein each of Q and Q' independently represents a residual group of an aromatic hydroxycarboxylic acid optionally substituted with an alkyl group, an aralkyl group, or an alkyl group and an aralkyl group; X is a counter ion; and M is a metal.
However, even the aforementioned metal oxide carriers have drawbacks in that when a developer containing a toner and the metal oxide carrier is used for a high speed copying machine in which the developer is rotated at a high speed, "a spent-toner problem" occurs in which the toner melted by heat caused by collisions between particles of the developer or between particles of the developer and the developing unit forms a film on the surface of the carrier particles. When this spent-toner problem occurs, the charging ability of the carrier deteriorates, resulting in toner scattering and background fouling of formed images.
To solve the spent-toner problem, developers having a resin-coated carrier have been disclosed. However, a good developer which avoids the spent-toner problem has not yet been obtained. For example, a carrier coated with a resin such as a styrene-methacrylate copolymer or a styrene polymer has good charging properties, but the critical surface tension of the surface of the resin-coated carrier is relatively high, which results in the spent-toner problem when the carrier is used for a long time. Therefore, the resin-coated carrier has a short life.
Toners include a charge controlling agent. Metal salts of monoazo dyes, nitrohumic acid and its salts, sulfonated copper phthalocyanine pigments, styrene oligomers having a nitro group or a halogen atom, chlorinated paraffin, melamine resins and the like are typically employed as negative charge controlling agents. These materials have a drawback in that they cannot be reliably manufactured because of their complex chemical structure. In addition, these charge controlling agents tend to decompose or undergo a change in quality when the toner containing these charge controlling agents is kneaded, resulting in deterioration of charge controlling ability of the resulting toner. Another drawback is that the charge controlling ability of the charge controlling agents can change depending on environmental conditions. Furthermore, if a toner that includes these charge controlling agents is used for a long time, the charging ability of the toner deteriorates, and a toner film is formed on a photoconductor, resulting in the deterioration of image quality.
Japanese Laid-Open Patent Publication No. 61-223753 discloses a toner that includes an aromatic hydroxycarboxylic acid metal salt such as a chrome complex of salicylic acid as a charge controlling agent. This toner has a drawback in that the charge of the toner varies substantially when environmental conditions are changed or if the toner is used for a long time.
Japanese Laid-Open Patent Publication No. 3-1162 discloses a toner that includes a fluorinated ammonium compound or a fluorinated iminium compound as the charge controlling agent. This toner has a drawback in that its charge is not stable when used together with a non-resin coated carrier. However, even though the toner is used with a carrier that is coated with a styrene-acrylate copolymer, the charge of the toner is seriously changed when the developer is used repeatedly even though being supplied with new toner. In addition, the toner has a poor charge rising property.
Japanese Laid-Open Patent Publication No. 6-337458 discloses a toner that includes a combination of a fluorinated ammonium compound or a fluorinated iminium compound and an aromatic hydroxycarboxylic acid metal salt as a charge controlling agent. This toner has a drawback in that the charge of the toner is not stable when used for a small developing unit, which is necessary for color image forming apparatuses wherein the toner receives a relatively heavy load per unit volume.
Because of these reasons, a need exists for a dry developer that can produce good quality images for a long time and maintain uniform charge properties even when used in a small developing unit.